This chapter describes structure and functions of normal lymph node, thymus and lymphoid hyperplasia. The outer region of the lymph node is just underneath the capsule of the lymph node. This area contains both primary follicles (inactive B-cell regions) and secondary follicles (activated follicles with germinal centers). Activated germinal centers contain predominantly B-cells (centrocytes and centroblasts), tingible body macrophages, follicular dendritic cells, and few T-cells. Major functions of lymph node are lymphopoiesis, filtration, and processing antigens. Major functions of normal thymus are antigen-independent t-cell maturation, negative selection of self-reactive clones, and positive selection of MHC-recognized clones. Some important topics are sinus histiocytosis with massive lymphadenopathy (Rosai-Dorfman disease), progressive transformation of germinal centers, vascular transformation of sinuses, bacillary angiomatosis, Kaposi’s sarcoma of lymph node, Castleman’s disease, dermatopathic lymphadenopathy, kimura lymphadenopathy (Kimura disease), and autoimmune lymphoproliferative syndrome (ALPS).